The present invention relates to a process for producing segmented polyurethanes having improved processing characteristics in the melt state and an efficient spinning process based thereon to form highly elastic, fine linear density elastane yarn and filament having improved mechanical and thermal properties by using hydroxyalkyl-terminated polysiloxanes in combination with specific crosslinking-capable polyisocyanates in the reaction of long-chain high molecular weight polyhydroxy compounds with organic diisocyanates and low molecular weight diols and also optionally further auxiliary and additive substances in the melt. The invention also relates to the elastane yarn obtained from this process by direct spinning and after a thermal treatment.
Elastanes are mono- or multifil continuous yarns composed of at least 85% by weight of segmented poly(urethane)s or poly(urethaneurea)s. The fibre-forming polymers have a segmented structure; that is, they are composed of xe2x80x9ccrystallinexe2x80x9d and xe2x80x9camorphousxe2x80x9d blocks (xe2x80x9chard segmentsxe2x80x9d and xe2x80x9csoft segmentsxe2x80x9d). The hard segments, owing to their xe2x80x9ccrystallinityxe2x80x9d, act as fixed points of the network and hence determine the strength of the shaped articles or fibres prepared from the polymers. By contrast, the soft segments, the glass transition temperature of which has to be below the use temperature, determine the elasticity of the elastanes.
Such elastanes are customarily prepared by polyaddition of long-chain dihydroxy compounds (macrodiols) with diisocyanates and low molecular weight dihydroxy or diamino compounds as chain extenders. High grade elastane filaments (also known as spandex) are prepared using poly(urethaneurea)s obtained by chain extension with diamines, since, compared with the diol-extended poly(urethane)s, they have (because of a larger number of hydrogen bonds between the polymer chains) a high hard-segment melting point on the one hand and excellent mechanical-elastic properties on the other. Elastane fibres are customarily produced by spinning solutions of these segmented poly(urethaneurea)s in highly polar solvents-such as dimethylformamide and dimethylacetamide by means of the so-called dry spinning process or the wet spinning process.
Because it does not require these relatively high boiling, aprotic solvents, a melt spinning process is preferable in principle to the aforementioned solution spinning processes for economic and ecological reasons. However, elastanes, for example from diamine-extended poly(urethaneurea)s, are not meltable without decomposition of the urea groups in the hard segments, as is discernible for example from Textilpraxis International 36, (1981) page 841. Thus, diamine-extended poly(urethaneurea)s are not spinnable from the melt. By contrast, the so-called thermoplastic poly(urethane)s having predominantly urethane hard segments which are obtained by chain extension with low molecular weight diols instead of diamines are spinnable into elastic yarn from the melt with a limited reduction in the molar mass. However, the use of such filaments is constrained by the fact that they (because of weaker binding interactions of their hard segments and the attendant lower softening temperature) do not withstand the thermal stresses arising in the course of the customary processing and treatment of elastanes. This applies in particular to the heat-settability of the elastic yarn for example in the course of the further processing with polyamide as hard fibre component into composite yarns at 195xc2x0 C. So the processing of commercially available thermoplastic poly(urethane)s does not lead to useful filament or fabric.
To raise the thermal stability of elastic yarn composed of thermoplastic poly(urethane)s, it is necessary to improve the binding interactions between the macromolecules, especially between the hard segments of the molecular chains of the polymers.
In prior art processes, this is accomplished in a particularly advantageous manner by using in the solvent-free synthesis of the polyurethane not only the conventional difunctional isocyanates but also at least one more highly functional polyisocyanate, which preferably contains isocyanurate groups, and directly spinning the PU reaction melt without further intermediate steps.
The European patent application bearing the file reference 95 105 479.0 describes such a process which compared with other previously known processesxe2x80x94described for example in DE-32 33 384 A1, EP 0 256 470 A2, DE-OS 16 69 402, EP 0 454 160 A2 and EP 0 397 121 A2xe2x80x94makes it possible to produce melt-spun elastanes which have virtually comparable mechanical-elastic and thermal properties to poly(urethaneurea) elastanes spun conventionally from solution.
However, the process described in EP-A 95 105 479.0 is in need of further improvements. It is in particular the experimental details reported in the Examples which reveal that the process mentioned falls short of ensuring the technical certainty needed to produce elastanes in consistently good quality, the desired diversity of types and with sufficiently high spinning efficiency.
On the basis of the experiments described in EP-A 95 105 479.0, the preference is for spinning yarn within the linear density range of 70-160 dtex from single hole jets. The spinning speed, where it is disclosed at all, is comparatively low at 100-500 m/min. Technically advantageous processes of elastane production would have to meet significantly higher requirements not only in the case of solution spinning but also in the case of melt spinning. The spinning of plural ends (4-8 ends) is well known in the art. It involves plural hole jets being used to spin in particular fine linear density monofils (20-50 dtex) typically at a spinning speed of 600-1000 m/min.
Reproducing and testing the recipes recited in EP-A 95 105 479.0 pointed up some deficits. The spinnability of the polyurethanes produced is limited in the molten state. Jet lives are limited and the spinning of fine linear density yarn (20-50 dtex) is possible only for a short period, if at all, under a plural end process regime. Efficient spinning at speeds of  greater than 500 m/min is not consistently achievable.
The invention has for its object to provide a process for producing elastic PU yarns which, compared with existing processes, shall have the following advantages in particular:
1. Poly(urethane) preparation and spinning should take place without the addition of a solvent.
2. Poly(urethane) preparation and spinning should take place continuously with a very short delay or residence time of the melt. In particular, it should be possible whilst abstaining from isolating the polyurethane intermediate in granule form to minimize the thermal stress and hence the possible degradation of the polyurethane.
3. Synthesis of the elastane raw material should take place in such a way that yarn spinning and aftertreatment does not give rise to any cleavage products for example in the form of NCO-capping agents.
4. The melt produced from the ingredients should not contain any relatively highly crosslinked constituents in the form of gel-like particles and should have time-invariant rheological properties to ensure consistent processing by spinning, making it possible to produce fine linear density elastane yarn having good mechanical and thermal properties even at high spinning speed.
This object is achieved by the hereinbelow more particularly described process according to the invention.
It was surprisingly found that elastane yarns having the required good properties and the process-based advantages mentioned are obtainable on using in the solvent-free synthesis of the segmented polyurethanes not only macrodiols but also selected polysiloxanes having hydroxyalkyl end groups and not only difunctional isocyanate compounds but also at least one more highly functional polyisocyanate and, following addition of low molecular weight diols as chain extenders, producing a reaction product in the molten state and spinning it directly without further intermediate steps.
The use of the crosslinking more highly functional polyisocyanates in combination with incorporable polysiloxane compounds leads to reaction products in melt form which have an advantageous set of processing characteristics for direct spinning. Simultaneous use of the components mentioned gives rise to considerably fewer gel particles capable of severely impairing the spinning process and the fibre""s quality especially in relation to the production of fine linear density yarn. The melt viscosity of the polyurethanes thus obtained can be lowered by using the polysiloxanes. As a consequence, melt spinning becomes controllable for a prolonged period. Pressure build-up across melt filtration means and pressure fluctuations in the region of the spinning jets are substantially ruled out.